Flexible wiring boards with a desired wiring pattern formed on a surface of an insulating film have been used in various types of equipment. Especially in recent years, flex-rigid wiring boards consisting of a mounting section on which electronic circuits are mounted and a flexible cable section extending from the mounting section have been used frequently in various types of electronic equipment.
As illustrated in FIG. 3, a flex-rigid wiring board 2 consists of a flexible section A in which cables are wired and a rigid section B on which circuits and electric elements are mounted. The flexible section A and the rigid section B are formed integrally with each other. Shields 4 for blocking external noise and preventing leakage of noise generated by wiring are attached to areas extending from near a boundary of the flexible section A and the rigid section B to an outer surface of the flexible section A. The shields 4 are made of a film over which a silver particle-containing conductive coating material is applied.
A method of manufacturing the flex-rigid wiring board 2 is as follows: First, a copper clad laminate 10 which is constituted by a base film 6 made of, for example, polyimide having sheets of copper foil 8 attached on both sides is provided. A through hole 11 is formed by, for example, drilling, at a predetermined position in the rigid section B, which is to be formed as the mounting section. Next, surfaces of the copper foil 8 and an inside surface of the through hole 11 are plated with copper to a predetermined thickness to form a plated through hole 12. The sheets of copper foil 8 which are plated are then etched through masks having predetermined circuit patterns to form circuit patterns in the flexible section A and the rigid section B, thereby providing wiring layers. The copper clad laminate 10 is then laminated, on both sides thereof, with insulating films 13 made of, for example, polyimide. Sheets of thin copper foil 16 are attached to the flexible section A and the rigid section B by thermo-compression bonding.
In this state, via holes 14 are formed by, for example, laser to reach the underlying wiring layers at predetermined positions. Then, in a desmearing process, smears as residues of the manufacturing process adhering to the via holes 14 and other places are removed with, for example, chemicals. The copper foil 16 and inside surfaces of the via holes 14 are then plated with copper. The sheets of copper foil 16 which are plated are then etched in the same manner as described above to remove the sheets of copper foil 16 in the flexible section A. Predetermined circuit patterns are formed in the rigid section B, thereby providing wiring layers.
Next, outer layer sides of the rigid section B are laminated with glass epoxy prepregs 18 and sheets of copper foil 20. The prepregs 18 are cured so that the sheets of copper foil 20 would be bonded to the rigid section B. Via holes 22 or other parts are formed at predetermined positions and the sheets of copper foil 20 are then plated. Predetermined circuit patterns are formed by etching in the same manner as described above and solder resists 24 are applied over the circuit patterns, i.e., over the outer surfaces of the rigid section B. Subsequently, the shields 4 over which a conductive silver coating material is applied are attached to areas extending from an end of the rigid section B to the flexible section A. In this manner, the flex-rigid wiring board 2 is completed.
As disclosed in Patent Document 1, there is also proposed a flex-rigid multilayer substrate in which a mounting section and a cable section have wiring layers of flex-rigid wiring boards of different thickness. Considering the fact that thickness of an insulating layer provided between a shielding layer and, for example, circuit wiring and a thickness and width of the circuit wiring affect the characteristic impedance of the circuit, the wiring in the cable section having a thin insulating layer between the wiring and the shielding layer is made thinner than the wiring of the mounting section in this flex-rigid multilayer substrate. In this manner, the mounting section and the cable section are consistent in the characteristic impedance.
Patent Document 2 discloses a structure of a flexible printed circuit board in which a circuit pattern or other copper foil in a mounting section of the flexible printed circuit board is thick so that the mounting section would be rigid and in which copper foil of wiring in a cable section is thin so that the cable section would be flexible.
Patent Document 1: Japanese Patent Application Laid-open No. H7-106766
Patent Document 2: Japanese Patent Application Laid-open No. 2007-250884